PORE SIZE DISTRIBUTION IN CRITICAL POINT AND FREEZE DRIED AGGREGATES FROM CLAY SUBSOILS

Using aggregates from 2 clay soils over a range of water contents from pF 1 to oven dry, shrinkage and water release curves were measured and pore size distributions found by mercury porosimetry after critical point and after freeze drying. Freeze drying caused less shrinkage, the maximum being 6 cm³ 100 g^?1 from pF 1, but gave a large increase, up to 10 cm³ 100 g^?1, of pores in the 0.1–10 μm size range. Critical point drying produced more shrinkage from pF 1, more than half of which was attributed to loss of interlamellar water and the rest to the collapse of pores larger than 10 μm. In these clays the volume of pores of over 3 μm diameter was very small (<3 cm³ 100 g^?1) and most of the plant available water was released by collapse of narrower pores and not by pore emptying.